WHICH TYPE OF GRAPH SHOULD YOU CHOOSE?

Transcription

1 PRESENTING GRAPHS

2 WHICH TYPE OF GRAPH SHOULD YOU CHOOSE?

3 CHOOSING THE RIGHT TYPE OF GRAPH You will usually choose one of four very common graph types: Line graph Bar graph Pie chart Histograms

4 LINE GRAPHS One of the most common graphs you will use is a line graph. Line graphs use a line to connect the data points that you plot. They are most useful for showing trends, and for identifying whether two variables are related to one another. An example of Trend data: How does temperature vary from month to month? An example of a Correlation: On average, how much sleep do people get, based on their age? You can only use line graphs when the variable plotted along the x-axis is continuous - for example, time, temperature or distance. Can you think of an example of a discontinuous variable?

5 BAR GRAPHS Bar Graphs are appropriate for data that are non numerical and discrete for at least one variable, i.e. they are grouped into separate categories. There are no dependent or independent variables. Important features of this type of graph include: Data are collected for discontinuous, non numerical categories (e.g. place, colour, and species) so the bars do not touch. Data values may be entered on or above the bars if you wish. Multiple sets of data can be displayed side by side for direct comparison (e.g. Males and females of the same age group).

6 BAR GRAPHS (CONTINUED) Axes may be reversed so that the categories are on the x axis, i.e. the bars can be vertical or horizontal. When they are vertical, these graphs are sometimes called column graphs (MS Excel uses this name for vertical bar graphs).

7 PIE CHARTS Pie Graphs can be used instead of bar graphs, generally in cases where there are six or fewer categories involved. A pie chart compares parts to a whole. As such it shows a percentage distribution. The entire pie represents the total data set and each segment of the pie is a particular category within the whole. So, to use a pie chart, the data you are measuring must depict a ratio or percentage relationship. You must always use the same unit of measure within a pie chart. Otherwise your numbers will mean nothing. Features of pie graphs include: The data for one variable are discontinuous (non numerical or categories). ie. graphing the number of 5 different species of fish. The data for the dependent variable are usually in the form of counts, proportions or percentages. They are not suitable for data sets with a large number of categories.

8 HISTOGRAMS Histograms are plots of continuous data and are often used to represent frequency distributions, where the y axis shows the number of times a particular measurement or value was obtained. For this reason, they are often called frequency histograms. Important features of this type of graph include: The data are numerical and continuous (e.g. height or weight) so the bars touch. (A column graph from Excel can be made into a histogram). The x axis usually records the class interval. The y axis usually records the number of individuals in each class interval (frequency).

9 COMPARING BAR GRAPHS AND HISTOGRAMS Notice that the bars are separated by spaces. We didn't have to do this... we could have had the bars touching, and this still would have been a bar graph. Note that the variables on the x-axis are discontinuous (there is a definite separation between blue and red, etc.) This graph is a histogram, but not just because the bars touch. In the bar graph example at the top of the page, you saw that bars in a bar graph can touch if you want them to... but they don't have to. Touching bars in a bar graph doesn't mean anything. In a histogram, however, the bars must touch. This is because the data elements we are recording are numbers that are grouped, and form a continuous range from left to right. There are no gaps in the numbers along the bottom axis; every possible fish mass can be located in one of the classes. This is what makes a histogram.